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      深入理解MPI实现
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        <p>本篇文章讨论MPI通信栈的各个层次以及制约MPI性能的主要因素。<br><a target="_blank" rel="noopener" href="https://dl.acm.org/doi/10.1145/3126908.3126963">(SC17) Why is MPI so slow?: Analyzing the fundamental limits in implementing MPI-3.1</a><br>这篇文章主要介绍了MPI-3.1标准的主要制约因素，对于深入理解MPI的实现很有帮助。其他的优化MPI的论文都是关注于MPI的典型实现（例如在MPICH或者OpenMPI里优化），而这篇文章则关注MPI-3.1标准的优化，并提出了一个基于MPI-3.1标准的高度优化的MPI实现，工作在MPICH的环境下实现的，主要工作是重新设计了MPICH通信栈里的ch4，如下图所示。</p>
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<p>MPICH是一个典型的多层次的软件栈。在MPI的用户接口层下面，最上层是“MPI Layer”，执行的都是机器无关代码，往下走是机器相关代码，也被叫做抽象设备层或者设备层，旧的MPICH提供两种设备：CH3（对于大多数平台）以及PAMID（IBM的蓝色基因）。CH4是本文重新设计的设备层，包括CH4 Core和Shmmods（共享内存模式通信）以及Netmods（网络互连模式通信）。一个MPI_PUT操作的控制流如下：（1）在MPI Layer层中检查参数错误、检查描述了访问进程和内存的MPI窗口对象、通信应该使用线程安全路径还是非安全路径；（2）CH4 Core检查局部性，如果目的进程就是自己，CH4 Core直接处理，如果通信发生在同一个节点上，就采用Shmmods，否则采用Netmods，控制流进入模块；（3）在Netmods或者Shmmods中分析操作，并判断是否可以直接在硬件实现还是要基于活动消息回退（没搞懂这个是在干嘛）。比如MPI_PUT操作，在Netmods模式下硬件能直接完成的操作仅限于：连续内存数据的RDMA操作，对于复杂的数据类型会回退到CH4 Core中的活动消息回退处理模块（active-message fallback）。在硬件上完成MPI_PUT操作时会将MPI级别的参数转化为网络级别的参数（比如将MPI中的目标偏移量转化为操作系统的虚拟地址），然后根据对应的MPI操作来完成一个或者多个网络操作。</p>

      
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